To create additional datasets for a BE, use the zfs command. The new dataset is located hierarchically under the root dataset of the BE.
This new dataset will not be automatically cloned to existing, associated zone BEs. Also, because the new dataset is under the root dataset of a BE, the new dataset will be private to that BE. The new dataset will not be a shared dataset.
After the dataset has been created under the root dataset, if you clone that entire BE, the new dataset would be included in the new, cloned BE.
For example, to create a new, non-shared dataset mounted at /myfs for the BE1, you would type:
$ zfs create -o canmount=noauto rpool/ROOT/BE1/myfs
For further information, see the zfs(8) man page.